Continuous tube rolling method and mandrel assembly for the implementation thereof

ABSTRACT

A method for rolling tubes on a continuous tube rolling mill involves deforming a tube billet using a mandrel assembly. The mandrel assembly includes cylindrical mandrel configured such that its ends can be alternately coupled to an attachment of the mandrel assembly. The mandrel is configured such that its ends have portions with a beveled lateral surface, wherein the angle of the generatrix of said surface to the longitudinal axis of the mandrel is between 10 and 70 degrees. The deformation of tube billets is carried out until the amount of wear on the working regions at both ends of the mandrel is not less than 25% of a critical value, then the mandrel is sharpened. The invention increases the service life of a mandrel, reduces the formation of flaws on the inside surface of the tubes, and prevents accidents from occurring during the rolling process.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. national stage application of aninternational application PCT/RU2017/000296 filed on 5 May 2017,published as WO/2018/208184.

FIELD OF THE INVENTION

The present invention relates to the industry of tube production (alsoknown as pipe manufacturing), in particular to the production of tubes(pipes) on a continuous tube rolling mill and the design of a mandrelassembly therefor.

BACKGROUND OF THE INVENTION

Continuous tube rolling mills with a floating long cylindrical mandrelhave been known since 1885 (patent of Canada No. 24956, published on 13Sep. 1886). For rolling tubes on the continuous mills, a floatingmandrel is used, it is inserted into a tubular billet (hollow shell),and when rolling the mandrel moves with rolled material beyond thecontinuous mill, then the mandrel is removed from the rolled materialusing a chain-type mandrel extractor. In this case, the mandrel is mademonolithic, with a guide cone at the front end, a cylindrical part and atail part in the form of an annular groove, for which a mandrelextractor grip is hooked.

Based on the necessity to expand the range of rolled tubes throughincreasing outer diameter and reducing mandrels weight, continuoustube-rolling mills with a retained mandrel were developed, they becamewidespread since the 60th of last century (patent of Great Britain No.1092718, IPC B21B 17/04, B21B 21/00, published on 11 Nov. 1967).

A mandrel assembly with controlled movement during rolling is used forrolling tubular billets (hollow shell) on continuous tube rolling millsof two- and three-roll type (MPM, PQF, FQM). The retention rate of themandrel assembly during rolling may be in the range from 500 to 2000mm/s depending on rolled tube grades, equipment design features andselected parameters for rolling. Extracting the mandrel from the rolledmaterial is carried out with a mill-extractor, which is located on thesame line with the continuous mill. The mandrel assembly consists of acylindrical mandrel with a guiding taper section at the front end, whichopposite end is connected to an extension element through a nippleconnection, and in the same way the extension element is connected to ashank end (the rear end part of a mandrel assembly) to retain themandrel in a gripper during rolling (U.S. Pat. No. 3,120,139, B21B25/04, published on 4 Feb. 1964).

When rolling with a mandrel assembly, hollow shells are deformed by acylindrical mandrel, other elements of the mandrel assembly relate to amachine-tool attachment and may include one or more extension elements,a shank end, a plug fitting, various inserts, bushings and fixtures.

The working part of the mandrel assembly is operated under complextemperature conditions during the production process and is subjected toconstant cyclic alternating loads, which intensify its wear from levelof which the quality of finished tubes is depended. A significantproblem in the tube production on units with a continuous rolling millis a short service life and a high cost of mandrels, which are mostlyimported.

The critical value of mandrel wear is regarded as the mismatch ofgeometric parameters and the state of a mandrel surface to specifiedrequirements. The critical value of mandrel wear is specified intechnological instructions of tube manufacturers. The mandrels are takenout of service when exceeding the wear critical value that includesvarious parameters, for example: occurrence of sections where the wearvalue at an outer diameter is above the specified one, occurrence of thevariety of surface inconsistencies in the form of “comets” on themandrel surface, mechanical damage, rough cracks formed as a result oftemperature influence, as well as when mandrel surface roughness exceedsthe specified critical values.

When the critical value of the mandrel wear is reached, the averagespecific mandrel consumption ranges from 0.1 to 4.0 kg/ton. The specificconsumption is regarded as the difference between weight of a wornmandrel before regrinding and mandrel weight after regrinding, referredto the number of rolled tons of products.

The disadvantage of the method of producing tubes on a continuous millwith the use of a mandrel assembly of built-up construction is thatduring rolling the mandrel wear is occurred at 200÷7500 mm from thefront end, whereas the opposite part of the mandrel practically does notwear out. However, when there is the critical value of mandrel wear, themandrel is sent to regrinding, during which the metal is removed overthe entire mandrel surface, which significantly reduces its operatinglife.

The most similar technical solution considered as the prototype for themethod and the mandrel assembly is the production of tubes on acontinuous tube-rolling mill using a mandrel assembly (patent of theRussian Federation No. 2486976, IPC B21B 25/00, published on 7 Jul.2013), in which a cylindrical mandrel is used. At the ends of a mandrelcylindrical part there are blind openings for connecting the cylindricalpart alternately to the extension element and the conical part of themandrel.

However, when producing tubes, as a result of deformation, rolled metalflows into the gap between the conical and the cylindrical mandrelparts, and various types of defects are formed on the inner tubesurface. In addition, it is possible that emergencies occur on acontinuous mandrel mill and a extractor-mill, for example, stopping therolling process because of equipment breakage, failure of protectingdevices, full contact of all the mandrel with the rolled metal, theformation of unfinished product. Also the prototype does not regulatethe value of the mandrel wear, which requires the necessary replacementof the end connection of the mandrel with mandrel assembly attachment,which can cause pre-schedule mandrel shutdown.

BRIEF SUMMARY OF THE INVENTION

The technical problem solved in the Invention is to extend the servicelife of the mandrel through increasing the number of rolled tubes on thesame mandrel, improving tubes surface quality and eliminating emergencysituations during rolling.

The specified problem is solved due to that the inventive continuoustube rolling method includes deformation of a tubular hollow shell usinga cylindrical mandrel with identical coaxial blind openings at the ends,control of mandrel dimensions and mandrel regrinding. The mandrel isconnected with the mandrel assembly attachment alternately to deform thetubular hollow shell by working sections from different ends of themandrel.

According to the Invention, the deformation of the tubular hollow shellis carried out until the wear value is at least 25% of the criticalvalue for one end of the mandrel working section. However, the mandrelregrinding is carried out after changing the end connection of themandrel with the mandrel assembly attachment, after subsequentdeformation of the tubular hollow shells and the wear of the workingsection on the other mandrel end is at least 25% of the critical value.

The specified problem is also solved due to that the mandrel assembly ofa continuous tube rolling mill, comprising a cylindrical mandrel that ismade with a beveled side surface at one end, has identical coaxial blindthread openings at the mandrel ends for an alternate connection throughjoining the mandrel to the mandrel assembly attachment according to theInvention, the opposite mandrel end is made with a beveled side surface,the angle of the beveled surfaces generatrix with the longitudinalmandrel axis at the both ends is the same and is in the range from 10 to70 degrees, whereas the opening at the front mandrel end is plugged atthe working position.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is illustrated by the attached drawing, whichschematically shows a mandrel assembly for the implementation of themethod of continuous tube rolling. The mandrel assembly includes acylindrical mandrel 1 with a beveled side surface 2 at both ends and amachine-tool attachment in the form of an extension element 3 and ashank end 4. At both ends of the cylindrical mandrel 1 identical blindthread openings 5 are made for connecting the mandrel to the extensionelement 3 alternately by means of, for example, a nipple connection. Atthe working position, the opening at the mandrel front end is pluggedwith the plug fitting 6 to protect from external influence and preventfrom clogging of scale and lubricant materials during rolling. Themandrel assembly can be connected with the shank end 4 also by means ofa nipple connection.

DETAILED DESCRIPTION OF THE INVENTION

During producing tubes on a continuous tube rolling mill, the wear of asection of the mandrel cylindrical part occurs mainly closer to thefront mandrel end, whereas the mandrel section near to the oppositemandrel end is not almost subjected to the wear. The deformation of thebillets (hollow shells) is carried out until the wear value of theworking mandrel section on the one end is at least 25% of its criticalvalue. When the wear value does not exceed 25% of the critical value,satisfactory condition of the mandrel surface and operational life ismaintained, but when such mandrels are removed from a mandrelscirculation system of the continuous rolling mill and when they arereground to a smaller diameter, a significant amount of metal is grindedand the mandrels average specific consumption increases from 2 to 40times. In addition, the pre-schedule removal of mandrels from operationsignificantly increases tool cost and adversely affects the performanceof tube-rolling equipment as a whole.

After the working mandrel section is worn away at one end, other mandrelend is connected to the machine-tool attachment, the plug fitting isinserted into an opening at the front mandrel end and hollow shells aredeformed. Furthermore, during the next operation cycle rolling iscarried out on the unworn working mandrel section until the wear valuereaches at least 25% of the critical value at the section on the othermandrel end. After wearing of the cylindrical mandrel is at least 25% ofcritical value almost along its entire length, the mandrel can bereground to other outer diameter along its entire length, which favor toincrease the service mandrel life, taking into account the regulation ofits wear value and improve the quality of finished tubes.

The mandrels wear value is regulated in the technological documentationof the enterprise and depends, in particular, on deformation modes on acontinuous rolling mill, on power parameters of the process, on therange and steel grade of rolled tubes, on the temperature of rolledmetal and rolling tools, on lubricants properties and antioxidants fordifferent tubes grades.

The mandrel is designed in such a way that the angle of the beveledsurfaces 2 generatrix with the longitudinal mandrel axis at the bothends is the same and is in the range from 10 to 70 degrees, whichensures to insert the mandrel easily into a hollow shell before rollingin a continuous rolling mill and favors the reduction of the number ofdefects on the inner tubes surface during the insertion and removal ofthe mandrel, as well as during rolling. If the generatrix angle is lessthan 10 degrees, there can be difficulties when the mandrel is insertedinto a hollow shell and when the extractor takes a mother tube out, inaddition defects on the inner surface of the rolled material are formedin the form of scratches and imprints. If the generatrix angle is morethan 70 degrees, the possibility of cracks formation increases at themandrel ends and in areas of blind openings, designed to connect withthe machine-tool attachment, which reduces the reliability of theconnection. In addition, after regrinding of worn mandrels outerdiameter to a smaller size, the angle of the beveled surfaces generatrixremains in the specified range, difficulties do not occur when themandrel is inserted into a hollow shell and there is practically noformation of defects on the inner surface of a hollow shell.

At the working position, the opening at the front mandrel end is pluggedwith the plug fitting 6 to protect from external influences and preventclogging with scale and lubricant materials during rolling. Threadconnections of all the elements of the mandrel assembly are madecomparable to the connection of casing tubes and have a taperedtrapezoidal thread or, for example, a Buttress thread. Before screwing,a lubricant is put on threads. Such constructive design of the mandrelassembly allows increasing the mandrels service life, the quality of theinner tube surface and preventing accidents during rolling.

Preferred Embodiment of the Invention

The proposed method for the continuous tubes rolling with the use of themandrel assembly is as follows. In the rolling process, the mandrelassembly is inserted into a hollow shell and then, together with thehollow shell, is set into a continuous mill. The hollow shell isdeformed into a mother tube strictly on the cylindrical mandrel part,located between two sections with beveled side surfaces. The wear of thecylindrical part section of the mandrel is formed mainly closer to thefront end, and the section of the mandrel near to the opposite endpractically does not wear out. After the front section of thecylindrical mandrel is worn at least 25% of the critical wear value, theplug fitting is removed from the opening at the front end of themandrel, the mandrel is disconnected from the attachment, connected bythe other end with the attachment and the plug fitting is inserted intothe opening at the other mandrel end. The operation of connecting anddisconnecting components of the mandrel assembly can be carried out on amachine for screwing and unscrewing mandrels. After that, during thenext cycle, the rolling process is carried out on the unworn section ofthe mandrel until it reaches a critical wear value at least of 25%.After wear of the cylindrical mandrel almost the entire length of atleast 25% of the critical, the mandrel can be reground along its entirelength to another outer diameter, which increases the service life ofthe mandrel and improves the quality of the finished tube. Whenimplementing this method for rolling tubes, the mandrel is easilyinserted into a hollow shell and removed, which leads to a significantreduction in the number of defects on the inner surface of the rolledmaterial.

When the pre-schedule removal of mandrels from operation, for example,when the wear value does not exceed 25% of the critical value, and itssubsequent regrinding to another outer diameter, the operationalmandrels life is significantly reduced, since in this case the averagespecific mandrels consumption increases from 2 to 40 times. Furthermore,the service life increases due to the possibility of using a plugfitting made without a thread, which reduces thread wear in the threadopenings due to a smaller number of screwing and unscrewing operations.

INDUSTRIAL APPLICABILITY OF THE INVENTION

The proposed method for the tubes production was tested on a PQFcontinuous tube mill. During industrial rolling processes, mandrels withthe diameter of 149.20÷179.55 mm and with an angle of 25 degrees for thebeveled surfaces generatrix at the both ends were used. Tubes wererolled using the 190 mm PQF mill calibration system. The mostsignificant mandrels wear occurred at the section located at a distanceof up to 5000 mm from the front mandrel end. The mandrel section,located at a distance from 5,000 and up to 11,500 mm (rear end) from thefront end, was almost not subjected to wear. After the front mandrelsection was worn by a value equal to 75% of the critical value, the plugfitting was removed from the opening at the mandrel front end, themandrel was joined to the extension element by another end, inparticular, on an equipment for the mandrel screwing on and unscrewing,then, the mandrel was put into operation again. Thereafter, the mandrelwas operated until its other end working section was worn at 75% of thecritical value. During the mandrel operation, the wear occurred alongthe entire length of the working sections; both rolling cycles wascharacterized by stable operation of the mandrel assembly, withoutdamage to the mandrel and threaded connections. On the tubes innersurface, the depth of single defects of various types did not exceedcritical values, the tubes quality was satisfactory. As a result of theperformed industrial rolling processes, the mandrels service life wasincreased by an average of 45.6%, emergency situations did not occur ona continuous rolling mill and mill-extractor.

The use of the proposed method for continuous tube rolling with the useof the mandrel assembly reduces tool cost by increasing its servicelife, reducing the formation of various types of defects on tubes innersurface, and eliminating the risks associated with emergency situationsduring the rolling process.

1. A continuous tube rolling method comprising the steps of: providing amandrel assembly of a continuous tube rolling mill, said mandrelassembly is capable of connecting to a mandrel assembly attachment; saidmandrel assembly includes a cylindrical mandrel having a first end and asecond end and two working sections adjacent to the first end and thesecond end respectively; wherein the first end and the second end areprovided with identical coaxial blind thread openings, and both thefirst end and the second end are capable of alternately connecting thecylindrical mandrel to the mandrel assembly attachment through thecoaxial blind thread openings; providing tubular hollow shells forprocessing by the continuous tube rolling mill; connecting the secondend with the mandrel assembly attachment; deforming the tubular hollowshells on the cylindrical mandrel at the working section adjacent to thefirst end with continuous controlling of a wear size of the workingsection, while the wear size is not exceeding 25% of a predeterminedcritical wear value; disconnecting the second end from the mandrelassembly attachment; connecting the first end with the mandrel assemblyattachment; deforming the tubular hollow shells on the cylindricalmandrel at the working section adjacent to the second end withcontinuous controlling of a wear size of the working section, while thewear size is not exceeding 25% of said predetermined critical wearvalue; and regrinding the cylindrical mandrel for further use.
 2. Amandrel assembly of a continuous tube rolling mill, said mandrelassembly includes a cylindrical mandrel capable of connecting to amandrel assembly attachment; said mandrel assembly defines a centrallongitudinal axis thereof; said a cylindrical mandrel has a first endand a second end; wherein: the first end and the second end are madewith a beveled side surface; identical coaxial blind thread openings areprovided on the first end and the second end both capable of alternatelyconnecting the cylindrical mandrel to the mandrel assembly attachment;wherein, when the first end is connected to the mandrel assemblyattachment, the second end is plugged with a fitting, and vice versa;and the beveled side surfaces each forms an angle with the longitudinalaxis ranging from 10 to 70 degrees.